This invention relates to a digital time base corrector. The purpose of this equipment is to accept the output from a video tape recorder (VTR) and to correct for errors in timing of the output signal with reference to a stable synchronizing signal.
The incoming video signal is converted into a digital format by analogue to digital converter (ADC). The digital output from the ADC is stored in a store which will conveniently have a capacity sufficient to store one or more complete television lines. The output from the store is converted into an analogue video signal by a digital to analogue converter (DAC).
The incoming video signal will be converted into a digital format at a clock rate determined by the input oscillator. The input oscillator will be caused to follow the timing perturbations of the output signal of the VTR. Such perturbations are caused by changes in the velocity of the tape transport and the scanning mechanism of the VTR.
The information read from the store into the DAC is under the control of an output oscillator which is generally locked to a local synchronizing signal.
The principle of the operation is that the information is written into the store at a speed which depends upon the perturbations on the output from the VTR but that the information is read out of the store at a constant rate. Thus the equipment eliminates time perturbations occurring on a video signal.
This particular invention relates to the improvements in the input oscillator which may be made in such a system. Generally the above configuration is known and recognised by engineers versed in the art of television systems. The improvements relate in particular to the method by which the input oscillator is caused to follow the perturbations on the video tape recorder output.
In a known configuration an oscillator design to track the information coming from the video tape recorder may take the form of a phase locked loop (PLL). The information coming from the video tape recorder contains synchronizing pulse information and colour sub-carrier information which corresponds to the standard television format. At the beginning of each line a synchronizing pulse is provided so that the commencement of the line is accurately known.
When the format is associated with a colour television system such as the
or NTSC systems, a colour sub-carrier burst is provided in addition to the synchronizing pulse.
The burst is typically ten cycles at colour sub-carrier frequency. This burst of sub-carrier occurring just after the synchronizing pulse on each television line is an indication of the reference phase which allows the colour information contained in the following picture period of the video waveform to be accurately decoded into its component parts.
A conventional input oscillator system for the digital time base corrector needs to provide a source of clock pulses capable of driving the ADC and store at a rate which is derived from the frequency of the synchronizing pulses and colour subcarrier information. During the course of one television line a digital time base corrector may take several hundred samples along the line. The method by which these sampling pulses may be generated is in a phase locked loop.
A voltage controlled oscillator (VCO) running at a suitable sampling frequency produces an output which is used to drive the ADC. The output is also applied to a system of multipliers and dividers which reduces the frequency to precisely that of the television line frequency. The output of the system is applied to one input of a phase discriminator which has its other input coming from the separated line synchronising pulses from the VTR.
The phase discriminator will produce a variable voltage at its output which is so arranged to control the frequency of the voltage controlled oscillator in a manner which will cause the phase error at the two inputs to the discriminator to be close to zero.
The above system is known and recognised by users of digital television equipment and represents only a description of one method currently being used. The disadvantage of such known equipment is that the instantaneous frequency of the VCO is a function of the output of the phase discriminator over several preceding lines. Therefore when a phase error between the off-tape signal and the VCO occurs, there is a finite response time before the error can be reduced to zero. For some types of off-tape timing perturbations it is impossible to design a phase locked loop which satisfactorily tracks the off-tape signal (that is to a close enough timing accuracy).
One method of overcoming the disadvantages of the known arrangement is provided by the invention described in the copending U.S. patent application Ser. No. 558,091 filed on the Mar. 13, 1975 by A. D. Stalley and J. A. Coffey filed concurrently herewith in which use is made of a triggered oscillator which is switched on in timed relationship with the video sync pulse and/or sub-carrier colour burst.
However, whilst the triggered oscillator operating in the mode described with reference to FIG. 3 or FIG. 5 in the specification of copending application referred to above, provides an accurate starting point for the input oscillator at the beginning of each line, it does not provide any correction for the change in frequency which may occur from one line to the next since this oscillator has a fixed frequency of oscillation.
The effect of velocity errors in the VTR is to cause an effective change of television line length which may be a small fraction of a .mu.second. The change of line length gives rise to an effective change of colour rendition as the line scans from the left hand side of the screen to the right hand side of the screen.